A projection exposure apparatus is used as an apparatus to form a circuit pattern of a semiconductor integrated circuit or a liquid crystal substrate on a semiconductor wafer in accordance with a photolithography technique. In such a projection exposure apparatus, a reticle (mask) is irradiated with an illuminating light beam radiated from an illuminating system so that an image of a reticle pattern is formed on a photosensitive substrate through a projection optical system. Such an apparatus is required to have highly accurate image formation characteristics in order to form a fine circuit pattern. Further, in order to expose an identical area on a substrate with a plurality of patterns to be overlaid thereon, such an apparatus is required to have a high degree of overlay accuracy between a layer to be subjected to exposure process and a layer having been previously subjected to exposure process. On the other hand, the projection optical system comprising a group of a plurality of lens elements suffers change in image formation characteristics such as magnification depending on ambient temperature. Accordingly, the apparatus is required to have stability regardless of ambient temperature in order to maintain the highly accurate image formation characteristics and the overlay characteristic as described above. For this reason, the projection exposure apparatus has been hitherto installed in a temperature-controlled constant temperature chamber. In order to prevent the projection exposure apparatus from adhesion of dust or the like in the chamber, for example, a chamber of the so-called down flow type is adopted, in which temperature-controlled air is allowed to flow from a ceiling of the chamber in a direction parallel to an optical axis of the projection optical system.
Now, a step-and-scan exposure method has been contrived, in which a slit-shaped rectangular area for a projection optical system is illuminated to perform exposure while relatively scanning a reticle and a photosensitive substrate. FIG. 12 schematically shows a scanning type exposure apparatus based on the use of this exposure system. This apparatus principally comprises an illuminating optical system (not shown) including a light source for irradiating a reticle R with a uniform illuminating light beam, a reticle stage RST for moving the reticle R in a scanning direction (X direction), a projection optical system PL for projecting an image of a pattern formed on the reticle R onto a wafer W at a predetermined reduction magnification, and a wafer stage WST for moving the wafer W in synchronization with the reticle R. In such an arrangement, the reticle R is illuminated with the uniform illuminating light beam collected through a condenser lens 3. The reticle stage RST, on which the reticle R is placed, is moved in the scanning direction with respect to an illumination area on the reticle R. In synchronization with the movement of the reticle stage RST, the wafer stage WST, on which the wafer W is placed, is moved in a direction opposite to the direction of the movement of the reticle stage RST. In accordance with the scanning for the reticle R across the slit-shaped illumination area, an exposure area on the wafer W is successively exposed with the reduced image of the pattern on the reticle R, formed through the projection optical system PL. According to this system, an area having a wide areal size can be exposed without enlarging the field size of the projection optical system. Moreover, only a part of the projection optical system is used for exposure. Therefore, this system is more excellent than other exposure systems in that it is easy to adjust, for example, distortion and uniformity of illuminance.
This apparatus has a chamber 1 which is a chamber of the down flow type as described above. Temperature-controlled air flows in a direction indicated by arrows in FIG. 12 from an air-blowing port 2 provided on a ceiling of the chamber 1. However, this apparatus includes the reticle stage RST which moves reciprocatively in the horizontal direction (X direction) over the projection optical system PL. Therefore, the way of flow of air going toward the projection optical system PL greatly changes depending on the position of the reticle stage RST. On the other hand, for example, a laser beam source for an interferometer, which serves as a heat source, is usually arranged in the vicinity of the projection optical system PL. Therefore, variation occurs in the temperature of the projection optical system PL itself, and in the temperature of air on an optical path between the reticle R and the projection optical system PL. As a result, a problem arises in that the image formation characteristics of the projection optical system PL change. The exposure apparatus of this type comprises the interferometer 6 for observing a reflected light beam from a movement mirror 5 fixed on the reticle stage RST in order to measure the position of the movable reticle stage RST. If the air flow in the vicinity of the reticle stage RST suffers variation, temperature-dependent fluctuation occurs in air on an optical path of the light beam radiated from the interferometer 6. Therefore, an error occurs in measurement of the position of the stage. As a result, a synchronization discrepancy is brought about in the movement of the reticle stage RST and the wafer stage WST. Further, the movement of the reticle stage RST also causes variation in reticle temperature as well as adhesion of dust or the like to the reticle R. These problems are inherent in the scanning type exposure apparatus comprising the reticle stage which is moved for the purpose of scanning.
Usually, as for projection exposure apparatuses of the collective exposure type and the scanning type, a projection optical system is supported by a pedestal fixed on a base plate of an apparatus, through a flange or the like provided on a barrel of the projection optical system. A wafer stage for holding a wafer (photosensitive substrate) and moving it in a scanning direction is installed in a space inside the pedestal. An optical path of an interferometer for irradiating a movement mirror installed at an end of the wafer stage with a laser beam and measuring a distance on the basis of a reflected light beam thereof is also included in the pedestal. A projection exposure apparatus has been hitherto known, which includes, in the space inside the pedestal, an air-conditioning system exclusively used for the inside of the pedestal, for supplying air having the same temperature as a temperature of an air-conditioning system used for the entire apparatus.
However, a laser beam source for such an interferometer, an electric substrate, and a control box are placed on a top plate of the pedestal. They may serve as heat sources in the apparatus. Further, the projection optical system supported by the pedestal also generates heat because the illuminating light beam is transmitted therethrough. Therefore, the surface temperature of the top plate of the pedestal is higher than a set temperature in a constant temperature chamber by about 0.5.degree. to 1.5.degree. C. Accordingly, in the space inside the pedestal, a portion of the space located inside with respect to the top plate of the pedestal, which contacts with the top plate of the pedestal, has a temperature higher than a temperature of central and lower portions of the space inside the pedestal. As a result, a temperature gradient is produced in the internal space of the pedestal. The temperature gradient causes temperature-dependent fluctuation in air (variation in refractive index) on the optical path of the interferometer. Thus an error of about several tens of nm is produced in a distance-measuring result concerning the wafer stage measured by the interferometer. The distance-measuring error of the interferometer causes an error concerning an irradiation position of a shot area on the wafer. The distance-measuring error also arises a problem of synchronization error in movement of the reticle stage and the wafer stage in the case of the scanning type projection exposure apparatus, which seriously affects image formation characteristics of an image of a reticle pattern formed on the wafer.
The projection exposure apparatus includes an alignment system for certainly executing overlay exposure. The alignment system includes a highly accurate and highly sensitive optical system such as an alignment microscope for detecting an alignment mark on the photosensitive substrate. Accordingly, the alignment system tends to be affected by the temperature-dependent fluctuation on its optical path, caused by the heat source such as an electric substrate as described above.
In the conventional apparatus, those which serve as the heat source such as an electric substrate, a light source box, and a control box are arranged outside the apparatus, or installed in a floating state to be separated from a main apparatus body. Alternatively, a local heat-exhausting mechanism is provided for such a heat source. Thus it is intended to suppress and minimize the influence of heat generated by the heat source, on the distance-measuring system and the alignment system.
However, those which serve as the heat source such as an electric substrate, a power source box, and a control box include those which should be arranged in the vicinity of a sensor in order to reduce noise, and those which should be installed on the main apparatus body, such as a laser head of a laser interferometer, in order not to deviate a relative position with respective to a measurement objective. Therefore, it is impossible to install all heat sources outside the apparatus, or install them in a floating state to be separated from the apparatus.
Even when a local heat-exhausting mechanism is provided for a unit which serves as a heat source, as provided in the conventional apparatus, the heat generated by the heat source warms air and components located in the vicinity of the heat source. Therefore, change in temperature and unevenness in temperature occur in a unit required to be precisely temperature-controlled, and in ambient air in the vicinity thereof. Thus results of measurement obtained by the distance-measuring system and the alignment system are badly affected due to fluctuation of air.